The quantitative assessment of functional activity of skeletal tissues has been the subject of developments over many years of exercise oriented research: Many global whole body responses to exercise, particularly the body oxygen utilization, heart rate and power output. Invasive procedures of localized oxygen/pH plus needle biopsies and quick freezing provide insight on localized muscle function. Phosphorous Nuclear Magnetic Resonance (31P NMR) has provided both global and localized evidence of biochemical responses to muscle activity, particularly the rise of adenosine diphosphate and inorganic phosphate as a function of exercise level during exercise, and of recovery times thereafter, particularly in vascular disease. Optical measurements of muscle function originated in 1935 with studies of hemoglobin/myoglobin changes in the cat soleus muscle on a relative basis. Progress on quantifying Hb/Mb measurements enabled as the phenomenon of photon migration was recognized: separation, absorption and scattering properties of exercising muscle were made possible by application of the diffusion theory. This research emphasizes the application of quantitative tissue oximetry to muscle exercise in terms of Hb desaturation and blood concentration characteristic of the gamut of performance from dystrophies to elite. Furthermore, initial transients of muscle oxygenation (warm-up), together with the dynamics of recovery, will be quantified as they appear to a remarkable extent in mitopathies. Finally and most important, imaging of functional muscle is possible with simple multi-source multi-detector combinations using continuous light and as, the physiology may require, using frequency and time domain optical techniques. Thus, images of Hb/Mb deoxygenation and blood concentration in muscle function will be available. The population to be studied ranges from elite Olympic performers (skaters, sprinters, endurance performers) through the normal sedentary population to the geriatric population with special emphasis on exercise protocols for the mitochondrial myopathy and CHF populations. In sum, the two major obstacles of optical studies of the limb may be overcome in these studies, namely quantitation and localization. Furthermore, the application to medically needful populations where outcomes can be altered by quantitative localized studies is consonant with the goals of "Healthy People 2000".